Manufacture of maleic anhydride from crude maleic acid



Fatented ha iv, was g aaia ag This repenture by a ass is at all times maintained in the e operation may he a continuous, seiniiii as or strictly batch process, one s 1 a method or" purify rig conveniently, owns sally and Without substan 'ai by p mation, crude rnaleic is obtained by the oxidaor a icy-product of the inanriiactrue phthalic ans, in the still during the distillation. hydride by the Gihbs-Conover process The maintenance of the liquid phase n States Patent #l,285,il7)- and the simultanependent upon a number of factors, notably t t; ous conversion of the acid thus obtained to antemperature within the vessel, the capacity of hydride. the equipment to transfer heat from the sou ce this application is a continuation-in-part thereof into the charge, the pressure within of my copending application S. N. 514,711, filed vessel, the quantity and nature of the impurit February 9, 1931, which in turn is a continuain the crude maleic acid and other factors. tion-in-part of my copending application S. N. With practice, the liquid phase can be main- 439263, filed March 26, N36. tained Without difficulty. In general, other cont is Well known that maleic acid undergoes ditions being favorable, the temperature should inter-molecular rearrangement forming fumaric be maintained as low as possible While mainzo acid when it is exposed to an elevated temperataining the presence of a liquid phase in an ture. This phenomena interferes with the ordiamount sufiicient to gain the advantage of a nary distillation o ma e o ac d, pa t cula y heat transfer from the wall of the vessel to crude maleic acid which contains organic and the solid maleic acid through a liquid phase no c pu it e In n effo t to d st rather than directly from the vessel to the solid 25 maleic acid (probably in the form of its anhyacid, so dride and water vapor) rapidly from the zone With most types of crude acid, both extremes of hea n a as low a mperature as possible in temperature as well as pressures are to be and her y re f maric a i rm t I avoided, Thus, even when heating the vessel hEVQtI'i d 0 d l m c d under as greatly vigorously, it is possible to reduce the pressure 30 leduced Dressllrs as P b With t u t sufiiciently to effect total disappearance of the th a small amount maleic acid, largely in liquid phase, notwithstanding the fact that there t m of maleic anhydride w t wat r, pass s is at all times an excess of maleic acid in solid di fi y from solid Vapor p a e as phase. After the liquid phase has once disthat the remaining material is in a d y Condiappeared, the heat transfer to the solid material 35 @1011 impairs the heat transfer, po the a is reduced to such an extent that local overt pr l d h t p a p t a! heating and the formation of fumaric acid ex- Qverheating and renders the Process of Prat" seeds the bounds of commercial practicability. Vague Facilities and conditions for improving the rate To overcome these disadvantages it has been of heat transfer from the vessel to the char e,

49 proposed (United States Patent #1,424,138) that including adeauate agitating means, large ex-= 95 t acid be l in h presence of an posed heating surface, as well as others known mart non-aqueous hqmd medmm whereby the to those skilled in the art, are to he water distills on as formed with the inert q q liquid, leaving the anhydride dissolved or susvarious pressing and temperatur" conlmnns a, 45 pended, after which it is separated therefrom by Wm be found depeflldmg 23 5 1. g distillation. This process suffers the inconvenstance m extent upon P j j' "j lenses of relatively large quantities of a iorctbeing treated, the nature 0:. the equipeign constituent as well as the need of extensive em a H equipment for the refining of a comparatively When the Vapors resulting i fi 59 small output of product. lation are condensed in a single receiving vessel 35 In my copending application, S. N. 514,711, one obtains a pure maleic acid product. It is I have disclosed and claimed a method of reprobable, as pointed out in my copending applifining maleic acid by distillation in the absence cation, that the vapor mixture which is conof an brganic solvent whereby a good recovery densed consists of water vapor and vapors of 55 of pure maleic acid may be had without suhmaleic anhydride and that the anhydride com- 119 bines with the water condensate to form maleic acid.

According to the present invention, maleic anhydride is obtained, usually in admixture with a small amount of maleic acid, by effecting the distillation while at the same time fractionating the vapors whereby the water is eliminated preferentially from the charge during the initial stages of the distillation leaving a charge which is enriched progressively iii maleic anhydride. As the distillation proceeds the vapors entering the fractionating column become richer in anhydride and by cutting the distillate during the course of the distillation it is possible to obtain a pure fraction which is rich in maleic anhydride, the remainder being maleic acid. The proportion of acid and anhydride in the product will depend upon the efliciency and extent of the fractionation. The safeguards against fumaric acid formation should be observed as in the case of a single distillation in which comparatively less fractionation occurs, particularly during the early stages of the distillation when water more or less free from acid is being eliminated from the batch and the batch is rich in acid. As the distillation proceeds and the charge contains a substantial amount of maleic anhydride in admixture with maleic acid, the dimculty incident to maintenance of a liquid phase becomes increasingly less diflicult. Moreover, the tendency to form fumaric acid is largely obviated and the dangers of fumaric acid formation and the difficulties incident to distillation disappear as the charge undergoing distillation becomes richer in anhydride.

In actual practice, and when a pure anhydride is desired, it is advantageous to-eflfectthe distillation in two stages; the first stage being one in which a partly dehydrated acid product is obtained and the various impurities present in the crude mixture are more or less completely eliminated, the second in which the dehydration of the residual acid present in the product is completed and a pure anhydride is obtained. However, if desired and particularly if the crude product contains comparatively small amounts of impurities, the distillation of the crude product and recovery of the anhydride may be performed in a single operation.

The following examples will serve to illustrate embodiments of my invention.

Example I.Maleic acid is heated to approximately 110 C. in a receptacle provided with an agitating mechanism adapted to be operated under reduced pressure conditions: As this temperature is attained, at least a portion of the acid will liquefy and the charge as a whole attains a mushy consistency. The distillation is effected through a fractionating column and under reduced pressure conditions of 5060 mm.

while maintaining the temperature of the batch and, if necessary, adjusting the pressure and temperature to maintain the liquid condition of the charge. large extent upon the nature and amount of impurities present. As the water is eliminated the danger of fumaric acid formation, as well as solidification of the charge is diminished. After an amount of water is eliminated which indicates that 75% or more of the acid has been converted to anhydride, the subsequent distillate is collected in a separated receptacle and the-distillation is continued until substantially all of the charge is volatilized.

Example II.-Proceed as set forth in Example Such adjustment will depend to a- I but first introduce an amount of maleic anhydride from a preceding batch equivalent to 20- 30% of the crude product. This heel will serve as a suitable flux for the crude acids and thereby minimize the difliculties incident to the maintenance of a liquid phase during the early stages of the distillation while water is being eliminated from the system.

Example III .-An enamel, steam jacketed kettle, which is provided with an agitator and a reflux column, is charged with a crude material containing -95% maleic acid. The charge is heated by means of steam or other heatingmedium within the jacket, and after a liquid phase appears it is slowly agitated. If desired, only a small amount of crude acid need be added to the still until a liquid phase appears, after which additional acid is added while at the same time the agitating mechanism is operated. After all of the acid has been added, the temperature in the jacket is adjusted to that of steam at 25-30 lbs. pressure '(l30-135 C.) and the pressure within the kettle is reduced to 30-50 mm. or lower if desired. If, because of the nature of the crude or the design of the equipment, a tendency toward the disappearance of the liquid phase becomes pronounced, the pressure should be increased whereby the temperature rises sufficiently to assure the maintenance of the liquid phase during the distillation. Water will be evolved during the early stages of the distillation in preponderance, while the charge undergoing distillation becomes proportionately richer in anhydride. After most of the water has thus been eliminated by fractionation the receiver is changed and the anhydride is collected as in the preceding example.

Example IV.--A distillate such as is obtained in the preceding example and which contains substantial amounts of maleic anhydride, is fractionally distilled at 20-30 mm. to eliminate substantially all 'of the combined water, which is present in the form of maleic acid, until the acid is converted substantially completely. to anhydride, after which the receiver for the distillate is changed and a maleic anhydride prodnot containing better than 98% maleic anhydride is recovered.

Although I have illustrated several embodiments of my invention, it is to be understood that the invention is not limited to the precise pressure and temperature conditions specified but, on the contrary, contemplates the simultaneous distillation and dehydration in the absence of a foreign organic solvent of maleic acid while maintaining a liquid phase in the still by sub-atmospheric pressure and temperature control.

What I claim is:

1. The method of obtaining a distillate rich in maleic anhydride from crude maleic acid, said distillate being essentially free from impurities present in the crude maleic acid, that comprises fractionally distilling the crude maleic acid gradually to drive off the combined water from the acid to form maleic anhydride while retain ing the unconvertedacid, the distillation being effected in the absence of an organic solvent and in the absence of a chemical dehydrating agent at a temperature not substantially in excess of 135 degrees centigrade and under sub-atmospheric pressure conditions while maintaining a liquid phase of the charge in the still by subatmospheric pressure and temperature control,

mosses the fractional distillation being continued until a substantial amount oi the acid has been converted into the anhydride and a corresponding amount of combined water has been eliminated, continuing the distillation to drive of! the fraction rich in maleic anhydrlde and collecting this fraction separately from the initial fraction rich in water,

2. The method of obtaining a distillate rich in maieic anhydride from crude maleic acid, said distillate being essentially free from impurities present in the crude maleic acid, that comprises i'racticnally distilling the crude maieic acid gradually to drive cfi the ccmbimed water from the acid is form maleic'anhyclricle while retaining the unconverted acid, the distillaticn being eflected in the absence c3 an organic solvent and in. the absence ci a chemical dehydrating agent at a temperature 110-3 subs'tan- MIC Ne @VORNIZKGW. 

